Is it possible to have a plane with its normal as reference for Transform Orientation
You can create custom transform orientations. The shortcut is &#9096; Ctrl&#9095; AltSpace or in the properties sidebar in the Tranform Orientations panel you will find a small + that will create a new orientation based on the current selection, in this panel you can also rename and delete any custom transforms you have created.

Once you have defined a custom transform you can activate it in the transform orientation menu so that your transform manipulator is oriented the way you want.